Conventional transient blocking units (TBUs) typically have a fixed voltage threshold or a fixed current threshold, which if exceeded, will cause the TBU to “trip” and switch into a substantially non-conducting state, thereby protecting following devices and circuits from the above-threshold transient.
A TBU may have a current limit that is too low for some applications to prevent nuisance tripping, or may be too high to allow sufficient thermal dissipation. A TBU has to be able to pass the normal operation peak currents without tripping, but must still effectively trip when sustained or very high peak currents occur. Such conventional TBU operation can be problematic in applications where large (but not damaging) transients can occur in normal operation. For example, transients can occur as the telephone receiver is lifted off the hook, the so-called “ring trip” condition. Substantial over-design and/or reduced levels of protection can occur if a conventional TBU is required to accommodate such normal transients without disconnecting.
TBU's using very small die package sizes designed to handle high power without encountering thermal problems can be difficult to accomplish. Further, high-resistance and dual TBU configurations are known to exacerbate the problem of very high transients, where high trip levels can create difficult conditions for TBU dissipation, often requiring additional heat-sinking to the device and consuming valuable package space.
Accordingly, there is a need to develop a TBU that can provide a high current limit to prevent nuisance tripping and further provide a low current limit that will protect following devices and circuits from the above-threshold transient.